Exploration 30 changes?

My question is pretty simple, yesterday I was averaging double the amount of the secret letters on exploration. I normally clear to wave 10, and it's somewhere between 13 to 20, today I didn't get more than 8 in a single run, I even got only 4 in some runs, anyone else noticed that or maybe I just got super unlucky? I find it hard to believe that I was just unlucky cause never before I got only 4 or 5 secret letters in a run